This invention relates generally to xerographic imaging employing multiple development and or process steps that result in significant variations in tribo charge between the independently developed toner images. More particularly, a method and apparatus is described for more efficiently transferring the toned images from a charge retentive surface to a substrate such as plain paper. Examples of multiple toner xerographic imaging include the multipass, image on image, full color systems and the tri-level, highlight color imaging schemes.
In the multipass, image on image system as many as four toners are sequentially developed either adjacent to or on top of each other before they are simultaneously transferred to the substrate in a single transfer step. As the multiple toner image is constructed on the charge retentive surface such as a photoreceptor, as many as three of the toners are exposed to up to three corona charging events by the photoreceptor charging device. The fourth and or last toner on the other hand is not. Since the corona charging history of the toners varies dramatically it is not surprising to find significant differences in their tribo values prior to transfer.
In tri-level, highlight color imaging, unlike conventional xerography, the image area contains three voltage levels which correspond to two image areas and to a background voltage area. One of the image areas corresponds to non-discharged (i.e. charged) areas of the photoreceptor while the other image areas correspond to discharged areas of the photoreceptor. Similarly to the multipass example, the tribo of the toners developed in these charged and discharged areas are significantly different and in this case of opposite polarity.
The tri-level, highlight color imaging example is used to describe the merits of this invention. The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
When using both DAD and CAD process simultaneously, the photoreceptor is developed with both color and black and color toners having negative and positive charge, respectively. Since the transfer corona device used in tri-level imaging has only one polarity, positive for example, just one of the images, color for example, would transfer to the final substrate. To deal with this situation, a corona discharge device is positioned immediately following the second developer housing, in this case the black developer housing, but preceding the transfer corona device. The function of the pretransfer corona device is to apply a corona of either negative or positive polarity so as to reverse the polarity of one of the two developed images, for example the black image.
While pretransfer corona reverses the sign of one of the two toners, it also adds additional charge to the other toner that is of the same polarity as the pretransfer device. Sometimes this additional charge is enough to cause the charge on that toner to be too high. When the charge is too high, transfer becomes very nonuniform thereby exhibiting elevated levels of print mottle. Print mottle is the blotchiness that can be observed in solid areas and can be very objectionable, especially with blue toners. By decreasing the energy to the pretransfer device, the overcharging can be eliminated, but this would cause an under charged condition for the other toner resulting in reduced transfer efficiency of that toner. This problem occurs because after pretransfer the charge levels on the color and black toners are unequal and separated too far apart in tribo space. Only one or the other may fall within the tribo range that can be accommodated by a single transfer setpoint. The color toner for example has a charge level of about -26 uc/g. and the black about -16 uc/g. The 10 uc/g. delta is significant. Hence, to maximize the latitude, the charge on the black and color toners after the pretransfer treatment should be equal to one another or as close to equal as possible.
It is well known in the prior art to subject a developed image on a charge retentive surface to corona discharge prior to image transfer for various reasons. For, example, U.S. Pat. No. 3,444,369 issued on May 13, 1969 relates to a method and apparatus for the reduction of background in transferred xerographic copy. A developed toner image on a photoconductive surface is subjected to a low level corona discharge of a polarity opposite the charge on the toner particles overlying the image areas. The corona charge opposite the image areas will be repelled by the like sign, but highly charged image areas of the photoconductive surface to thereby render the image area toner unaffected. The corona charge opposite the non-image areas of the photoconductive surface will not be repelled and will thus convert the toner overlying the non-image areas to a polarity opposite that on the image area toner particles. This will permit the electrostatic transfer of the image area toner to the transfer media such as plain paper, but will tend to suppress the transfer of the non-image area toner.
It is also known to utilize light exposure and corona discharge prior to image transfer as shown in U.S. Pat. No. 4,506,971. In this device the light exposure occurs prior to the corona exposure. As stated therein, blurred images are minimized or eliminated in a xerographic reproduction prior to transfer by first exposing the image to light to at least substantially discharge the background around the image and to reduce the charge on the photoreceptor holding the image thereto. Secondly, a charge of opposite polarity of the charged photoreceptor is deposited onto the toned image and photoreceptor. This, as stated, produces a very stable image for transfer since a very strong holding force is produced to hold the image in place as the image enters the transfer station.
U.S. Pat. No. 3,784,300 issued on Jan. 8, 1974 relates to a copying apparatus with a pretransfer station including a pretranster corotron and lamp arranged such that the light exposure of the photoreceptor is subsequent and not simultaneous with the pretransfer corona charging.
U.S. Pat. No. 4,205,322 issued on May 27, 1980 relates to an electrostatic recording apparatus in which a toner image consisting of toner particles of at least two different kinds and of different polarities is efficiently and reliably transferred to a recording medium such as an ordinary sheet of paper. The toner particles having different polarities are all converted into those having one polarity and after such conversion the toner image (with its two kinds of particles) is electrostatically transferred to the recording medium, the transfer involving both kinds of particles at the same time.
U.S. Pat. No. 5,038,177 granted to Parker et al on Aug. 6, 1991 describes balanced, efficient corona transfer for both the charged area image and the discharged area image of a developed tri-level image is obtained by the provision of a selective pretransfer charge corona device in combination with a pretransfer discharge lamp. While improved transfer over prior art devices is obtained using a pretransfer lamp prior to pretransfer charging the preferred embodiment of the invention utilizes a pretransfer lamp before and in coincidence with pretransfer charging.